Capacitive sensors can be used for distance measurement and position detection. Various arrangements of the electrodes of such sensors create an electric field between the electrodes. As the electric field changes, a corresponding current change can be measured, and from the relationship between current and voltage the parameters of a coupling network between the electrodes can be determined.
An object that is brought or moves into the field between the electrodes can alter the properties of the field in a variety of ways. For example, the object can increase the coupling by coupling via a higher permittivity. The object can also increase the coupling by electrical conductance. Additionally, the object can decrease the coupling by causing a surge of part of the field to ground.
Using multiple electrodes, a kind of electrical tomography of the space in front of the electrodes can be produced. This sensing principle is currently well-known for the realization of seat occupant sensors for the front passenger seat of a vehicle. Such systems use a certain number of electrodes to generate an electrical field and at least one electrode to measure the electrical field generated by the other electrodes. Given the number of electrodes needed to implement these systems, the complexity and cost are relatively high. Additionally, the hardware needed to interface the electrodes with an electronic control unit (ECU) is a significant part of the system cost. Thus, extending a system for the three back seats of a vehicle using the same technology as for the front passenger seat could increase the overall system price, not to mention the complexity, by a factor of four or more, which is undesirable.